This invention relates to a method of producing reticular structures, and particularly, to the production of metallic reticular structures, as well as to a device suitable therefor.
Reticular structures made from metal and other materials have a wide range of application. For example, these structures can be used as lightweight structural components, battery plates, electrochemical anodes and cathodes, filters for fluids, separation devices for fluid media, heat shields, and for numerous other applications.
Numerous methods for producing such types of structures are known. Automated production of such reticulated structures, however, is extremely difficult to implement, primarily because, with the conventional methods, the reticulated foam bodies that serve as patterns or pre-structures must be bonded to wax plates. The step of bonding a foam pre-structure to a wax plate is almost impossible to automate. The bonding points are, however, indispensable, since it is through these points that the foam pattern is burned out and then, through the resulting junctions that the molten metal flows into the cavities or voids formed by the foam pre-structure.
U.S. Pat. No. 3,616,841 (Walz: issued 1971) which is viewed as the closest prior art, discloses a method for the production of an insoluble foam material with a predetermined reticulated structure. This method encompasses the steps of producing a self-supporting reticulated polyurethane foam; producing a refractory mold material by filling the voids of the polyurethane foam with a watery gypsum plaster suspension that then sets; heating the refractory mold material to a temperature of about 120° C. (250° F.) over a time period of two hours; producing voids in the refractory mold material by raising the temperature of the refractory mold material to between 535 and 815° C. (between 1,000 and 1,500° F.), in order to completely vaporize the foam and produce a mold; introducing a molten substance into the refractory mold in an amount sufficient to fill the voids which had been previously occupied by the reticulated foam pre-structure; solidifying the molten substance by reducing the temperature to below the melting point of the substance; and washing out the material that constitutes the refractory mold material. The molten substance comprises metals, metal alloys, ceramics and/or cermet.
The method disclosed by Walz has several disadvantages. The equipment required for melting the substance that is poured into the refractory mold is either very expensive, especially for melting high-melting-point metals, or is technically not feasible. Another disadvantage is that in an automated process it is very difficult to control the bonding of the foam to the wax plate. This step is critical, however, for controlling the quality of the final product, as the quality of the bonding between foam and wax plate determines the structure of the foam pre-structure, which, in turn, determines the technical parameters such as surface smoothness or dimensional accuracy, of the end product. Thus, in order to reliably obtain an end product that corresponds to specification with regard to surface smoothness or dimensional accuracy or other parameters, it is imperative that this step be controllable in order to restrict the statistical range of fluctuation in the structure of the foam as much as possible.
To form the reticular structure, molten metal is poured into the refractory mold, which consists of branched voids. With the Walz method, in order to ensure that the molten metal remains liquid long enough to flow through the branches and completely fill the voids, the mold material must be heated to a temperature higher than that of the melting point of the molten metal. As a result, the solidification of the molten metal progresses very slowly, resulting in a solidified metal with a coarse grainy texture and reduced strength properties.
To solve this problem, Walz suggests various cooling methods, such as, for example, spraying the mold with water or air. A problem with such cooling methods is that the mold hinders the flow of heat, thereby significantly diminishing the cooling effect. Moreover, the production of massive or solid areas of metal together with the reticular structure is related to the problem of a very slow cooling progress. In order to obtain a bubble-free and fine-grained texture, it is imperative that the solidification process of the reticular structure be a controlled process. The method steps disclosed in Walz do not provide a means for effective control over the solidification process. The Walz method has an inherent economic disadvantage that limits the success or feasibility of automating production processes for reticular structures, in that the slow progression of the solidification of the metal results in long process times.
What is needed, therefore, is an automated method of production of reticular structures, particularly, metallic reticular structures. What is further needed, is such a method that produces a reticular structure having a fine-grained and bubble-free texture. What is yet further needed is such a method that allows large-scale production of metallic reticular structures, including large-dimensioned reticular structures.